Lithium secondary batteries have good energy densities, output densities, and the like and are effective for achieving size and weight reduction. Therefore, the demands therefor as power supplies of portable appliances such as notebook personal computers, cellular phones, and handy video cameras are growing sharply. Lithium secondary batteries have also attracted much attention as power supplies for electric vehicles, load leveling of electric power, and the like. In recent years, demands as power supplies for hybrid electric vehicles are expanding rapidly. In particular, for electric vehicle applications, excellence is necessary in terms of low cost, safety, lifetime (at high temperatures in particular), and load characteristics, and improvements of materials have been desired.
Among materials constituting lithium secondary batteries, substances that have a function of allowing elimination and insertion of lithium ions can be used as positive electrode active materials. There are a variety of positive electrode active materials and each has its own characteristics. A common objective toward improvements of performance is improvements of load characteristics and improvements of materials are strongly desired.
Moreover, materials with good performance balance that are excellent in terms of low cost, safety, and lifetime (at high temperatures in particular) are desired.
Presently, lithium manganese-based composite oxides having a spinel structure, layer lithium nickel-based composite oxides, layer lithium cobalt-based composite oxides, and the like are put into practice as the positive electrode active materials for lithium secondary batteries. Lithium secondary batteries that use these lithium-containing composite oxides have advantages and disadvantages in terms of their characteristics. That is, lithium manganese-based composite oxides having a spinel structure are inexpensive and relatively easily synthesized and exhibit high safety when made into batteries on one hand, but have low capacities and poor high-temperature characteristics (cycle and storage) on the other hand. Layer lithium nickel-based composite oxides have high capacities and good high-temperature characteristics but have disadvantages such as that they are difficult to synthesize, exhibit poor safety when made into batteries, and require care for storage. Layer lithium cobalt-based composite oxides are easy to synthesize and exhibit an excellent battery performance balance and are thus widely used as power supplies for portable appliances but have significant disadvantages such as insufficient safety and high cost.
Under such current circumstances, lithium nickel manganese cobalt-based composite oxides having layer structures have been proposed as promising candidates of active materials that eliminate or minimize disadvantages of these positive electrode active materials and exhibit a good battery performance balance. In particular, as the demands for cost reduction, higher voltage, and higher safety increase in recent years, lithium nickel manganese cobalt-based composite oxides are regarded as promising positive electrode active materials that can meet all such demands.
In particular, one of the possible solutions that meet the demands for lower cost and higher capacity is to reduce the proportion of expensive cobalt, set the nickel/manganese ratio to about 1 or less, and use the battery by setting a higher charge voltage. However, when the charge voltage is set higher, a high load is applied on an electrolytic solution, which leads to problems such as generation of gas and deterioration of storage characteristics. An alternative for this is to reduce the proportion of expensive cobalt, set the nickel/manganese ratio to about 1 or more, and to use the battery without setting a high charge voltage. However, since lithium nickel manganese cobalt-based composite oxides having such a composition range are easily sinterable at a relatively low firing temperature, the productivity lowers and high crystallinity is not obtained. Thus, lithium secondary batteries that use these oxides as the positive electrode material have relatively low capacities and exhibit low output characteristics. Thus, further improvements are needed for practical application.
Heretofore, lithium nickel manganese cobalt-based composite oxides having manganese/nickel atomic ratios and cobalt ratios that fall within composition ranges corresponding to values defined by the present invention have been disclosed in Patent Documents 1 to 32 and Non-Patent Documents 1 to 73.
However, Patent Documents 1 to 32 and Non-Patent Documents 1 to 73 contain no descriptions that focus on additives that suppress growth and sintering of active substance particles during firing in the composition ranges defined by the present invention and do not satisfy prerequisites for improving battery performance in the present invention. It is extremely difficult for these technologies to solely achieve improvements of battery performance attained by the present invention.
There is no literature that describes “to suppress growth and sintering of active material particles during firing” indicated by the present invention. However, Patent Documents 33 to 47 and Non-Patent Documents 74 and 75 below have been published as known literatures describing that lithium nickel manganese cobalt-based composite oxides are combined or substituted with a compound containing W, Mo, Nb, Ta, or Re, or the like so as to improve the positive electrode active materials.
Patent Document 33 and Patent Document 34 disclose the use of W, Mo, Ta, or Nb as an element substituting transition metal sites of a lithium nickel-based composite oxide having a layer structure and that this improves the thermal stability in a charged state. However, as for the property of particles, only the size and the value of specific surface area of secondary particles of a system using Nb are described and there is no description about primary particles.
It is described that at a stage of adjusting the particle size of raw materials, the raw materials are wet-ground in a wet bead mill until the particle diameter is 1 μm or less. However, at such a grinding level, fine primary particles cannot be obtained by firing. In addition, since the main components of the composition is Li and Ni, there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Patent Document 35 discloses use of a lithium nickel manganese cobalt niobium-based composite oxide. However, as for the particle properties, the descriptions related to the size of the primary particles and secondary particles are not provided except for that the particles are substantially spherical and thus the control of particle size is not addressed. Furthermore, since the Mn molar ratio in the transition metal is significantly low, i.e., 0.1 or less, there remains a problem that an active substance that has well-balanced properties cannot be obtained still.
Patent Document 36 discloses use of a lithium nickel manganese cobalt-based composite oxide containing W, Mo and that this improves thermal stability in a charged state while achieving a lower cost and a higher capacity than LiCoO2. However, in Examples, because a Ni—Mn—Co composite oxide having an average particle diameter of 10 μm, lithium hydroxide, and tungsten trioxide or molybdenum trioxide are mixed with each other and fired, the reaction becomes inhomogeneous and diffraction peaks of a composite oxide of Li and W and/or a composite oxide of Li and Mo are contained in addition to the main diffraction peaks belonging to a hexagonal crystal structure. Moreover, the manufacturing method involves homogeneously mixing the Ni—Mn—Co composite oxide having an average particle diameter of 10 μm, lithium hydroxide ground to 20 μm or less, and 1 to 20 μm tungsten trioxide or molybdenum trioxide and firing the resulting mixture. A raw material mixture of such particle sizes does not correspond to a fine and homogeneous mixture and it is impossible, by firing such a mixture, to obtain a fired powder containing spherical secondary particles formed by aggregation of fine primary as in the present invention. Thus, there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Patent Document 37 discloses a lithium nickel manganese cobalt-based oxide having a layer structure in which Ta and Nb are used as the elements substituting transition metal sites, and that this achieves a wider operable voltage range, good charge/discharge cycle durability, high capacity, and high safety. However, the manufacturing method described in this document involves simply mixing a nickel manganese cobalt coprecipitate powder, a lithium compound, and a compound of Ta or Nb and firing the resulting mixture and is not a manufacturing method that takes into account the control of the particle morphology. Thus, it is impossible to obtain the spherical secondary particle morphology formed by aggregation of fine primary particles obtained in the present invention. Moreover, since a Ni—Mn—Co coprecipitate powder having an average particle diameter as large as 8 μm, a niobium oxide powder, and a lithium hydroxide powder are mixed and fired, the reaction becomes inhomogeneous. Thus there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Patent Document 38 discloses Examples in which transition metal sites of a lithium nickel manganese cobalt-based composite oxide are substituted with W. However, the manufacturing method described in this document is not a manufacturing method that takes into account the control of the particle morphology and it is impossible to obtain the spherical second particle morphology formed by aggregation of fine primary particles obtained in the present invention. Patent Document 38 also discloses Examples in which transition metal sites of a lithium nickel manganese cobalt-based composite oxide are substituted with W. However, the Mn molar ratio in the transition metal sites is as low as 0.01 and the Ni molar ratio is significantly large, i.e., 0.8. Thus there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Patent Document 39 discloses that a lithium manganese nickel-based composite oxide having a monoclinic structure with transition metal sites thereof substituted with Nb, Mo, W is used as a positive electrode active substance and that this can provide a lithium secondary battery having a high energy density, a high voltage, and high reliability. However, the manufacturing method described in this document is a manufacturing method that involves grinding and mixing raw material compounds in a ball mill or the like and firing the resulting mixture and does not take into account the control of the particle morphology. It is impossible to obtain the spherical second particle morphology formed by aggregation of fine primary particles obtained in the present invention. Moreover, since Co is not contained as the transition metal element, crystals do not develop sufficiently and the Nb, Mo, W molar ratio is excessively large, i.e., 5 mol %. Thus there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Patent Document 40 discloses that a compound having molybdenum or tungsten is provided to at least surfaces of lithium transition metal oxide particles having a layer structure and that good battery characteristics can thereby be exhibited in a more hostile operation environment. However, according to Examples described in this document, the manufacturing method involves heating precipitates containing Co, Ni, and Mn, mixing a lithium compound and a molybdenum compound thereto, and firing the resulting mixture. Thus, the effect of suppressing particle growth or sintering during firing of the positive electrode active substance is not easily exhibited and fine primary particle morphology of the present invention cannot be achieved. Moreover, according to Examples, since the Co/(Ni+Co+Mn) molar ratio is as large as 0.33, there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Patent Document 41 discloses a lithium nickel manganese cobalt molybdenum-based composite oxide having a layer structure. However, according to Examples described in this document, the manufacturing method involves grinding and mixing a lithium compound and a molybdenum compound with a Mn, Ni, Co coprecipitate hydroxide, i.e., mixing powder particles while grinding, and then firing the resulting mixture. Thus, it is impossible to obtain the spherical second particle morphology formed by aggregation of fine primary particles obtained in the present invention. Moreover, according to Examples, since LiOH is mixed with Mn—Ni—Co coprecipitate hydroxide and molybdenum oxide in a grinding mortar and the resulting mixture is fired, not only the reaction becomes inhomogeneous but also the Co ratio is high, i.e., the Co/(Ni+Co+Mn) molar ratio is as high as 0.34. Thus, there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Patent Document 42 discloses an active substance for a lithium secondary battery, in which a surface layer containing Li and at least one element selected from the group consisting of Mo and W is preferably provided on surfaces of lithium nickel cobalt manganese-based composite oxide particles. It is described that thermal stability can thereby be made higher than that of conventionally proposed positive electrode active substances without significantly deteriorating the high initial discharge capacity. However, the manufacturing method described in this document is a manufacturing method involving mixing a Li composite oxide of Mo or W with a positive electrode active material powder later and then re-firing the resulting mixture. Thus, not only the effect of suppressing particle growth or sintering during firing of the positive electrode active substance is not exhibited but also a state in which the Li composite oxide of Mo or W simply coats surfaces of the positive electrode active substance is created. Thus, the resistance at the active substance surface increases and there is a problem in that good output characteristics cannot be obtained still.
Patent Document 43 discloses that particles of a lithium composite oxide are contained as the positive electrode active substance particles, that at least part of secondary particles have cracks, that W, Nb, Ta, or Mo is at least provided to surface layer portions of the active substance particles, and these elements are distributed more in surface layer portions than in the interiors of the active substance particles. As for the particle properties, the document discloses that part of secondary particles have cracks, that the median diameter is 1 to 30 μm, and that the average particle diameter of the primary particles is 0.1 to 3.0 μm in general. However, each of the descriptions is nothing more than a description of a general range and there is no description defining the ratio between the primary particle size and the secondary particle size. Moreover, according to Examples described in this document, the manufacturing method involves adding additive elements to surface layer portions of lithium composite oxide particles by a post-treatment (a solution method, 400° C. annealing). Thus, not only the effect of suppressing particle growth or sintering during firing of the positive electrode active substance is not exhibited but also a state in which additive element-containing compounds simply coat the surfaces of the positive electrode active surfaces is created. Thus, the resistance at the active surfaces surface increases and there is a problem in that good output characteristics cannot be obtained still. Furthermore, since the method involves treating the aforementioned elements at a low temperature relative to the positive electrode active material so as to support these elements on the surfaces, only the additive elements are presumably present in the surface layer portions of the active substance and it is expected that a continuous composition slope structure that has a non-linear concentration gradient in the depth direction from the primary particle surfaces does not exist. Thus, there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Patent Document 44 describes that Mo or W is added to a lithium nickel manganese cobalt composite oxide but there is a problem in that good output characteristics cannot be obtained still because the primary particle diameter is as large as 1 μm or more.
Patent Documents 45 and 46 disclose use of a lithium nickel manganese cobalt-based composite oxide containing Nb, Mo, and W. However, in Examples, none of the three elements are used and only examples with a Mn/Ni molar ratio of 1 are implemented. Thus, there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Patent Document 47 discloses further adding Nb, W, and Mo to a lithium nickel manganese-based layer composite oxide containing Li in sites containing transition metals. However, not only Co is not contained but also only Examples with a Ni/Mn molar ratio of 1 or less are provided. Thus, there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Non-Patent Document 74 discloses a LiNi1/3Mn1/3Mo1/3O2 composite oxide having a layer structure. However, the Mo content is excessively high and Co is not contained. Thus, there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.
Non-Patent Document 75 discloses a Mo-doped LiNi1/3Mn1/3Co1/3O2 composite oxide. However, not only the Co/(Ni+Mn+Co) molar ratio is as large as 1/3 but also only compositions with a Ni/Mn molar ratio of 1 are described. Moreover, as for these compositions, the maximum firing temperature is low, i.e., 900° C. In addition, although an oxide (MoO3) is used as the Mo raw material, all other materials are acetic acid salts (water-soluble). As long as SEM images of Non-Patent Document 2 is concerned, primary particle size has grown to about 2 μm even at a low firing temperature of 800° C. Thus, there is a problem in that an active substance that strikes a good balance between various properties cannot be obtained still.    Patent Document 1: Japanese Patent No. 3110728    Patent Document 2: Japanese Patent No. 3571671    Patent Document 3: U.S. Pat. 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